Field of the Invention
The present invention relates to a surface acoustic wave resonator filter. More specifically, the invention relates to a band-pass filter of the type of a surface acoustic wave (SAW) resonator utilizing a longitudinal mode resonance of a surface acoustic wave and having a wide band characteristics.
An equivalent circuit of the conventional two pole mode SAW resonator is illustrated in FIG. 7A. It has been known that such SAW resonator includes two series resonating circuits 11 and 12 having frequencies f.sub.11 and f.sub.12. The phases of two resonating frequencies f.sub.11 and f.sub.12 are in opposite phase relationship. This opposite phase relationship is equivalently illustrated by a transformer 13.
When an impedance matching is established in the two pole mode SAW resonators under a certain condition, a filter characteristics as illustrated in FIG. 7B is obtained with a pass band between the resonance frequencies f.sub.11 and f.sub.12. A center frequency in the pass band is f.sub.l. As a typical filter employing this principle, a monolithic quartz filter has been well known. With such filter construction, each insertion loss at the resonance frequencies f.sub.11 and f.sub.12 becomes substantially consistent with 3 dB point attenuating 3 dB from a minimum insertion loss (generally 0 dB) of the pass band.
As conventional means for realizing such two pole mode resonator, there are a longitudinal mode resonation type for generating two resonance modes in a propagating direction of the surface acoustic wave and a transversal mode type for generating two resonance modes in a direction transverse to the propagating direction. This two pole mode resonator has been disclosed in M. Tanaka et al., "NARROW BANDPASS FILTER USING DOUBLE-MODE SAW RESONATORS ON QUARTZ" in the 38th Annual Frequency Control Symposium of IEEE, 1984, and M. Tanaka et al., "Narrow Bandpass Double Mode SAW Filter", of the 15th EM Symposium of Electric Communication Committee, 1986.
FIGS. 8A and 8B show the concrete construction of this two pole mode resonator. FIG. 8A shows a transversal mode coupling type filter, in which two resonators a and b are proximately arranged longitudinally perpendicular to the propagating direction of the surface acoustic wave. The resonator a has an IDT (interdigital) electrode 1a at a center and reflectors 3a and 4a at both sides of the IDT electrode 1a. Similarly, the resonator b has an IDT electrode 1b at the center and reflectors 3b and 4b at both sides of the IDT electrode 1b.
In this transversal mode coupling type filter, the band-pass filter characteristics as shown in FIG. 7B is obtained by a coupling of two modes (even symmetric mode illustrated by f.sub.1 and odd symmetric mode illustrated by f.sub.2 ).
FIG. 8B shows a longitudinal mode coupling type filter, in which two IDT electrodes 1 and 2 are proximately arranged in the longitudinal direction as the propagating direction of the surface acoustic wave. At both sides of these IDT electrodes 1 and 2, reflectors 3 and 4 are arranged respectively.
With this longitudinal mode coupling type filter, the band-pass filter characteristics as illustrated in FIG. 7B is obtained by the coupling of two modes generated between the input side IDT electrode 1 and the output side IDT electrode 2.
In such type of the surface acoustic wave filter, it is necessary to expand the interval between the resonance frequencies of two modes to be used in order to widen the filter characteristics. There is a restriction on the design parameter to cause various problems, such as large use impedance, degradation of the attenuation magnitude out of the band and so forth. Therefore, there is a limitation for widening the band.